1. Field of the Invention
The present invention relates to a method for producing an image which is stable to heat (heat resistant), stable to actinic light (light resistant), is not chemically reactive (chemically resistant), and is mechanically string (abrasion resistant). More particularly, it relates to a processing for converting a silver image formed in a silver halide photographic emulsion layer into a heat resistant, light resistant, chemically resistant and abrasion resistant binderless noble metal image.
2. Description of the Prior Art
A silver halide photographic emulsion layer is colored due to thermal decomposition of the binder when heated to about 150.degree. to 200.degree. C, therefore, it can not be used for purposes requiring heat resistance.
One of the fields which requires a heat resistant image is "super-microphotography." An image reduced on a 35 mm film from a 9 by 14 inch (23 by 36 cm) size original with a reduction ratio of about 10 is usually called a "microphotograph," and a further reduced image (about 2 by 3 mm) by a factor of about 10 is called a "super-microphotograph." That is, a microphotograph can be considered to be an image reduced by a factor of about 10 and a super-microphotograph to be an image reduced by a factor of about 100.
Since the image size of a super-microphotograph is about 2 by 3 mm or smaller, the enlarging factor is about 100 (10,000 based on area ratio) when a super-microphotograph is projected on a screen to provide the original image size. Consequently, a light intensity of about 10 million lux is necessary on the image surface of the super-microphotograph, if the image projected on a transmission type screen, e.g., with a blackened back surface, must have a light intensity of about 100 lux when the screen has a transmission density of 1. In fact, the super-microphotographic image is illuminated with a light intensity of about 12 to 13 million lux to compensate for the loss of the projection lens.
The temperature of the emulsion layer of a supermicrophotograph increases to several hundred degrees C due to the heat generated by the light absorbed in the emulsion layer, when the super-microphotograph is continuously illuminated with such a strong light. As a result, the binder of the emulsion layer is thermally decomposed and colored to cause the image projected on the screen to be dim and colored. Since the silver image areas absorb light well, the temperature of these areas preferentially increases and the binder in these areas is preferentially decomposed, then the decomposition spreads into the surrounding areas. Decomposition of even the binder at the non-silver image areas proceeds in an accelerated manner, once the binder is slightly colored and light absorption occurs.
It has been discovered previously that when an emulsion layer having a silver image therein is heated to about 400.degree. to 600.degree. C in an oxygen-containing gas, such as air, the black silver image turns to a metallic silver image with a mirror surface. The binder of the emulsion layer turns dark red brown due to thermal decomposition. In a subsequent heating process (hereafter, the heating process is designated "baking") in an oxygencontaining gas, the binder is decomposed into oxide gases (e.g., CO.sub.2, H.sub.2 O, and SO.sub.2) and removed and the optical density of the colored binder layer at the non-silver image areas gradually decreases. Also, the mirror surface of the silver image disappears probably due to the migration of silver particles (probably silver atoms), and at the same time the silver image loses the ability to resolve lines of a thickness of several microns even though the silver image resolves lines of about 1 micron before baking. It may be considered that the silver atoms migrate from their original location to different locations because minute silver crystals are found at the image areas and the periphery of the image areas and because many pinholes and cracks are observed in the silver image. Cracks and pinholes decrease the optical density of the baked silver image. In particular, the optical density of the high density areas is reduced to a great extent providing a partially reversed image. If the baking is continued until the decomposed binder is completely decomposed and removed, the silver image is seriously damaged and can not be used as a super-microphotographic image.